The present invention pertains to the field of apparatus for treatment of a failing heart. In particular, the apparatus of the present invention is directed toward reducing the wall stress in the failing heart.
The syndrome of heart failure is a common course for the progression of many forms of heart disease. Heart failure may be considered to be the condition in which an abnormality of cardiac function is responsible for the inability of the heart to pump blood at a rate commensurate with the requirements of the metabolizing tissues, or can do so only at an abnormally elevated filling pressure. There are many specific disease processes that can lead to heart failure with a resulting difference in pathophysiology of the failing heart, such as the dilatation of the left ventricular chamber. Etiologies that can lead to this form of failure include idiopathic cardiomyopathy, viral cardiomyopathy, and ischemic cardiomyopathy.
The process of ventricular dilatation is generally the result of chronic volume overload or specific damage to the myocardium. In a normal heart that is exposed to long term increased cardiac output requirements, for example, that of an athlete, there is an adaptive process of ventricular dilation and myocyte hypertrophy. In this way, the heart fully compensates for the increased cardiac output requirements. With damage to the myocardium or chronic volume overload, however, there are increased requirements put on the contracting myocardium to such a level that this compensated state is never achieved and the heart continues to dilate.
The basic problem with a large dilated left ventricle is that there is a significant increase in wall tension and/or stress both during diastolic filling and during systolic contraction. In a normal heart, the adaptation of muscle hypertrophy (thickening) and ventricular dilatation maintain a fairly constant wall tension for systolic contraction. However, in a failing heart, the ongoing dilatation is greater than the hypertrophy and the result is a rising wall tension requirement for systolic contraction. This is felt to be an ongoing insult to the muscle myocyte resulting in further muscle damage. The increase in wall stress is also true for diastolic filling. Additionally, because of the lack of cardiac output, there is generally a rise in ventricular filling pressure from several physiologic mechanisms. Moreover, in diastole there is both a diameter increase and a pressure increase over normal, both contributing to higher wall stress levels. The increase in diastolic wall stress is felt to be the primary contributor to ongoing dilatation of the chamber.
Prior art treatments for heart failure fall into three generally categories. The first being pharmacological, for example, diuretics. The second being assist systems, for example, pumps. Finally, surgical treatments have been experimented with, which are described in more detail below.
With respect to pharmacological treatments, diuretics have been used to reduce the workload of the heart by reducing blood volume and preload. Clinically, preload is defined in several ways including left ventricular end diastolic pressure (LVEDP), or left ventricular end diastolic volume (LVEDV). Physiologically, the preferred definition is the length of stretch of the sarcomere at end diastole. Diuretics reduce extra cellular fluid which builds in congestive heart failure patients increasing preload conditions. Nitrates, arteriolar vasodilators, angiotensin converting enzyme inhibitors have been used to treat heart failure through the reduction of cardiac workload through the reduction of afterload. Afterload may be defined as the tension or stress required in the wall of the ventricle during ejection. Inotropes such as digoxin are cardiac glycosides and function to increase cardiac output by increasing the force and speed of cardiac muscle contraction. These drug therapies offer some beneficial effects but do not stop the progression of the disease.
Assist devices include, for example, mechanical pumps. Mechanical pumps reduce the load on the heart by performing all or part of the pumping function normally done by the heart. Currently, mechanical pumps are used to sustain the patient while a donor heart for transplantation becomes available for the patient.
There are at least three surgical procedures for treatment of heart failure: 1) heart transplant; 2) dynamic cardiomyoplasty; and 3) the Batista partial left ventriculectomy. Heart transplantation has serious limitations including restricted availability of organs and adverse effects of immunosuppressive therapies required following heart transplantation. Cardiomyoplasty includes wrapping the heart with skeletal muscle and electrically stimulating the muscle to contract synchronously with the heart in order to help the pumping function of the heart. The Batista partial left ventriculectomy includes surgically remodeling the left ventricle by removing a segment of the muscular wall. This procedure reduces the diameter of the dilated heart, which in turn reduces the loading of the heart. However, this extremely invasive procedure reduces muscle mass of the heart.
The present invention pertains to a device and method for reducing mechanical heart wall muscle stress. Heart muscle stress is a stimulus for the initiation and progressive enlargement of the left ventricle in heart failure. Reduction of heart wall stress with the devices and methods disclosed herein is anticipated to substantially slow, stop or reverse the heart failure disease process. Although the primary focus of the discussion of the devices and methods of the present invention herein relates to heart failure and the left ventricle, these devices and method could be used to reduce stress in the heart""s other chambers.
The devices and methods of the present invention can reduce heart wall stress throughout the cardiac cycle including end diastole and end systole. Alternatively, they can be used to reduce wall stress during the portions of the cardiac cycle not including end systole. Those devices which operate throughout the cardiac cycle are referred to herein as xe2x80x9cfull cycle splintsxe2x80x9d. Those devices which do not operate to reduce wall stress during end stage systole are referred to as xe2x80x9crestrictive devicesxe2x80x9d. Restrictive devices include both xe2x80x9crestrictive splintsxe2x80x9d which alter the geometric shape of the left ventricle, and xe2x80x9cwrapsxe2x80x9d which merely limit the magnitude of the expansion of the left ventricle during diastolic filling without a substantial shape change.
While it is desirable to reduce wall stress for the treatment of heart failure, to slow or reverse the disease process and to increase heart wall muscle shortening and pumping efficiency, it is also desirable to maintain or improve stroke volume and allow for variable preload.
Improving muscle shortening both total length change and extent at end systole, is particularly important in symptomatic heart failure wherein the heart has decreased left ventricle function and has enlarged. Full cycle splinting can be used to obtain a substantial increase in muscle shortening. Improved shortening will lead to an increase in pump function, and chronically may result in muscle strengthening and reversal of the disease because of increased pumping efficiency. The increase in shortening should be balanced against a reduction in chamber volume.
In asymtomatic, early stage heart failure, it may be possible to use only a restrictive device or method as elevated wall stress is considered to be an initiator of muscle damage and chamber enlargement. Restrictive devices and methods acting during diastole will reduce the maximum wall stress experience during end diastole and early systole. It should be understood that restrictive devices and methods can be used in combination with full cycle splinting to more precisely control or manipulate stress reduction throughout the cardiac cycle.